We propose to continue our study of specific transmembrane signalling based on the structure and behavior of the cholera toxin (CT). We will continue our high-resolution X-ray crystallographic structure analysis of the cholera toxin. We will initiate crystallographic studies of physiologically significant complexes designed to reveal a) the structural basis for the recognition and binding of the specific oligosaccharide component of the ganglioside receptor, GM1; b) the organizational principles that give a quasi 5-fold symmetric A1B5 aggregate; c) the binding and catalytic mechanism of the A subunit towards the substrates NAD and the arginine analogue guanyl tyramine. Using the molecular image derived from the X-ray studies we intend (in collaboration with Prof. R. Josephs) to study the disposition of the cholera toxin on (or in) the receptive membrane by electron microscopy and begin to probe the dynamics of the facilitated transfer of the catalytic subunit through natural and artificial membrane by radioactive iodination and susceptability to protease digestion. We will initiate a parallel study of crystalline exotoxin A from Pseudomonas aeruginosa which shares many physiological and biological features with cholera toxin.